The present invention is directed generally to optical communications, and particularly to a modulated optical transmitter.
Wavelength division multiplexing (WDM) is gaining widespread use in optical communications because, unlike conventional communications systems where a higher transmission capacity usually requires faster components, the transmission capacity of a single optical fiber may be increased simply by making more effective use of the available bandwidth, without requiring the use of faster components. Also, WDM permits signals at different wavelengths to be routed to different destinations.
WDM transmitters commonly use a number of independently controllable, fixed single frequency sources whose outputs are combined into a single transmitter output. WDM transmitters typically fall into two categories, namely those in which all optical channels are capable of being modulated simultaneously and independently, and those where a single modulator is used to modulate the output of one or more lasers. Transmitters in the latter category are described as being wavelength selectable. Wavelength selectable WDM transmitters advantageously require only a single RF modulator connection, pose less stringent requirements on the quality of the optical output from the light sources, and may be used with conventional packaging. Consequently, the wavelength selectable transmitter is one of the more commonly used optical communications sources.
An example of a wavelength selectable transmitter is disclosed in U.S. Pat. No. 5,394,489, issued to an inventor of the present application. In an embodiment of the disclosed invention, the output from an array of individually actuable semiconductor lasers is combined in a combiner integrated on the same substrate as the lasers. An amplifier amplifies the single output taken from the combiner, and the amplified output is subsequently modulated in a modulator.
Interferometric modulators, such as the Mach-Zehnder modulator, demonstrate several favorable characteristics such as low or zero chirp, high power handling capabilities and low insertion loss. However, interferometric modulators conventionally suffer from the disadvantage of taking up a significant amount of space. This problem is compounded when an amplifier is added to the transmitter. Other types of modulator, for example the electro-absorption filter, take up less space, although they do not offer the same favorable chirp, power handling and insertion loss characteristics.
Factors such as component size and system complexity are important considerations in the development of optical communications systems. In addition, optical losses are generally reduced along with the number of components, i.e. when the system complexity is reduced. There is therefore a need to develop an improved WDM transmitter where the transmitter is smaller, the overall transmitter complexity is reduced, and where transmitter has fewer optical losses.
Generally, the present invention relates to a modulated optical transmitter. An embodiment of the invention is directed to an interferometric modulator that include an optical amplifier or an attenuator as a phase modulation element. The modulator may include combinations of phase shifters, amplifiers and attenuators in each arm for overall control of the output power and depth of modulation.
Other embodiments of the invention are directed to a wavelength division multiplexed optical transmitter that includes a number of individual light sources operating at independent frequencies. Outputs from each of the light sources are combined in an optical combiner. In one particular embodiment, the combiner has two output ports, each feeding into respective arms of an interferometric modulator. In another embodiment, the combiner has four output ports, feeding into two parallel modulators whose outputs are then combined to form a gray scale signal. In another embodiment, the combiner has one output port feeding into a modulator having an amplifier or attenuators disposed in at least one arm.
Advantages of transmitting the output from two combiner ports directly into respective arms of the interferometric modulator include increasing the amount of light entering the modulator and avoiding optical losses associated with an input y-branch of the modulator. The resultant increase in light transmitted by the modulator reduces the need for an amplifier integrated in the transmitter. The reduction in the number of transmitter components also reduces fabrication and system complexity, and reduces the overall size of the device.
Advantages of including amplifier or attenuator elements in the modulator include the ability to optimize the modulator""s on/off ratio, and the overall control of modulator output power.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.